1. Field of the Invention
This invention relates to methods of adjusting certain oscillators to a standard, and to oscillators adapted to oscillate at a nominal frequency of 3375 kilohertz. Accordingly, it is a general object of this invention to provide new and improved methods and oscillators of such nature.
2. Description of the Prior Art
Precisely made quartz crystals are capable of controlling, with a high degree of accuracy, the frequency of various communications and related electronic equipment. For example, VHF and UHF transceivers using narrow bandwidths must maintain their frequency close to the center of their operating channel. Many of these transceivers incorporate frequency synthesizer techniques, and, for these, a standard frequency crystal oscillator of high precision is required. It is usually in the 1 to 5 MHz range and is frequency divided to some lower frequency related to the synthesized channel frequency spacing. For alignment of these transceivers, the standard frequency oscillator is set precisely; for non-synthesized transceivers, the frequencies are usually set with an electronic counter having as a time base a frequency of comparable high precision.
The alignment of such a frequency reference standard to the required accuracy has been made by varying a small trimmer capacitor in the crystal oscillator circuit until the frequency matches some standard source such as the radio transmissions of the National Bureau of Standards at 2.5, 5, 10 or 15 MHz or at the lower frequency of 60 kHz. Reception of the high frequency signals is subject to propagation disturbances which make comparison difficult much of the time, while use of a local standard based on the reception of the 60 kHz signal is of limited utility because of the complexity and generally high cost of the required low frequency receiver. Direct comparison with an atomic controlled frequency standard such as cesium or rubidium clock is also generally ruled out because of their cost. To surmount these difficulties, the National Bureau of Standards has determined that a standard frequency can be distributed over the country via the color television network system in the form of the frequency of the color subcarrier. Presently, network color programming originating in New York and Los Angeles contain a color burst frequency synchronized to such an atomic standard. Thus, a color television receiver receiving such programs can at the proper times be used as a frequency standard of high precision.
A technique for using this standard was developed at the NBS. Basically, it involves the construction of a high precision 5 MHz crystal oscillator and a second crystal oscillator at the color burst frequency of 3.5795454... MHz phase locked to the first, using frequency synthesizer techniques. Two oscillators are used because the color burst frequency f.sub.c is not an integral multiple of any simple frequency which might serve as a time base or frequency reference such as 1 kHz or even 1 Hz. The horizontal sweep frequency f.sub.h used in color telecast display is 15.734265...kHz and the vertical sweep frequency f.sub.v is 59.94... Hz. All these frequencies are non-integral, but are interrelated by simple fractions, and are derived from one another and a 5 MHz output of the atomic frequency standard. The relationships are: ##EQU1##
The use of television color subcarrier for adjusting local frequency standards to high accuracy has been recommended by the National Bureau of Standards [Electronics, May 10, 1971, pp. 96-98, and Mar. 20, 1975, pp. 107-112]. The author of such articles, Dicky D. Davis, is the patentee of U.S. Pat. No. 3,958,269, issued May 18, 1976, entitled "Color Subcarrier Frequency Comparator".
Using the frequencies suggested by the applicant of this invention, including 3375 kilohertz, phase-locked-loop synthesis and display-generating circuitry in the frequency comparison are rendered unnecessary. Adjustments of an oscillator can be performed with just a color-TV receiver.